[strongSwan] DH group MODP_2048 inacceptable, requesting MODP_1024

[Kevin Clark]

Been scratching my head over this for a couple of hours now.  Time for the experts to take a look ;-)

Everything was working fine with Ubuntu 10.04 (Strongswan 4.3.2).  A colleague has updated to Ubuntu 10.10 (Strongswan 4.4.0) and now we get:

[IKE] DH group MODP_2048 inacceptable, requesting MODP_1024

Here's the setup:

initiator: Strongswan 4.4.0 (Ubuntu 10.10)
responder: Strongswan 4.4.1 (Centos 5.5)

responder config:

config setup
	# plutodebug=all
	# crlcheckinterval=600
	# strictcrlpolicy=yes
	# cachecrls=yes
	 nat_traversal=yes
	 charonstart=yes
	 plutostart=yes

#conn %default
#    left=aaa.bbb.ccc.ddd
#    leftsubnet=111.222.333.444/24
#    leftid=@aaa.bbb.ccc.ddd
#    leftcert=aaa.bbb.ccc.ddd.crt
#    leftfirewall=yes

conn rw-linux-kclark
    left=aaa.bbb.ccc.ddd
    leftsubnet=111.222.333.444/24
    leftid=@aaa.bbb.ccc.ddd
    leftcert=aaa.bbb.ccc.ddd.crt
    leftfirewall=yes
    right=%any
    rightsourceip=192.168.100.0/24
    rightcert=initiator.crt
    auto=add

The Strongswan documentation indicates that MODP_2048 is supported through the GMP plugin, which is loaded:
 
[root at responder ~]# ipsec listalgs
---<snip>---
Status of IKEv2 charon daemon (strongSwan 4.4.1):
  uptime: 88 days, since Oct 22 18:56:34 2010
  malloc: sbrk 385024, mmap 0, used 229096, free 155928
  worker threads: 9 idle of 16, job queue load: 0, scheduled events: 0
  loaded plugins: aes des sha1 sha2 md4 md5 random x509 revocation pubkey pkcs1 pgp dnskey pem fips-prf xcbc hmac gmp attr resolve kernel-netlink socket-raw stroke updown eap-identity eap-mschapv2

And the MODP_2048 algorithm is registered:

[root at responder ~]# ipsec listalgs
---<snip>---
List of registered IKEv2 Algorithms:

  encryption: AES_CBC 3DES_CBC DES_CBC DES_ECB 
  integrity:  AES_XCBC_96 HMAC_SHA1_96 HMAC_SHA1_128 HMAC_SHA1_160 HMAC_SHA2_256_128 HMAC_MD5_96 HMAC_MD5_128 HMAC_SHA2_384_192 HMAC_SHA2_512_256 
  hasher:     HASH_SHA1 HASH_SHA224 HASH_SHA256 HASH_SHA384 HASH_SHA512 HASH_MD4 HASH_MD5 
  prf:        PRF_KEYED_SHA1 PRF_FIPS_SHA1_160 PRF_AES128_XCBC PRF_HMAC_SHA2_256 PRF_HMAC_SHA1 PRF_HMAC_MD5 PRF_HMAC_SHA2_384 PRF_HMAC_SHA2_512 
  dh-group:   MODP_2048 MODP_2048_224 MODP_2048_256 MODP_1536 MODP_3072 MODP_4096 MODP_6144 MODP_8192 MODP_1024 MODP_1024_160 MODP_768 

So why does the responder reject MODP_2048 when it is a supported algorithm?

Kevin